Recovering hydrocarbons from subterranean zones relies on drilling wellbores.
Wellbores are made using surface-located drilling equipment which drives a drill string that eventually extends from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. Drilling fluid usually in the form of a drilling “mud” is typically pumped through the drill string. The drilling fluid cools and lubricates the drill bit and also carries cuttings back to the surface. Drilling fluid may also be used to help control bottom hole pressure to inhibit hydrocarbon influx from the formation into the wellbore and potential blow out at surface.
Modern drilling systems make use of downhole probes. Downhole probes may comprise any active mechanical, electronic, and/or electromechanical system that operates downhole. A probe may provide any of a wide range of functions including, without limitation, data acquisition; sensing; data telemetry; control of downhole equipment; status monitoring for downhole equipment; collecting data by way of sensors (e.g. sensors for use in well logging) that may include one or more of vibration sensors, magnetometers, nuclear particle detectors, electromagnetic detectors, acoustic detectors, and others; emitting signals, particles or fields for detection by other devices; sampling downhole fluids; etc. Some downhole probes are highly specialized and expensive.
Downhole conditions can be harsh. Exposure to these harsh conditions, which can include high temperatures, vibrations, shocks, and immersion in various drilling fluids at high pressures can shorten the lifespan of downhole probes. Supporting and protecting downhole probes is important as a downhole probe may be subjected to high pressures (20,000 p.s.i. or more in some cases), along with severe shocks and vibrations. Replacing a downhole probe that fails while drilling can involve very great expense.
It is common to drill different sections of a wellbore using different-diameter drill bits. For example, the section of a wellbore closest to the surface may be drilled with a larger-diameter bit. The next part of the wellbore may be drilled with a smaller bit. The deepest part of the wellbore may be drilled with a still smaller bit.
Downhole probes as are used, for example, in directional drilling applications, measuring while drilling (MWD) applications, and/or logging while drilling (LWD) applications may be provided with centralizing fins intended to keep the probes centralized in the bore of the drill string. Where such a probe is used in drill string sections having bores of different diameters the fins may not always support the probe well with the result that the probe may suffer damaging vibration or impact with the drill string.
One solution to this is to change the centralizers when it is desired to use the probe in a different diameter of drill string. However, a probe may include several centralizers. Changing the centralizers can be labor-intensive, costly, and may require dismantling of the probe or parts of it. Dismantling the probe at the well site can lead to reliability issues.
In some prior probes centralizers comprise fins that can be trimmed to fit into drill string sections of smaller diameters. Trimming the fins is often done with a knife. This can be dangerous and also results in inaccurate sizing of the centralizer to the drill string section it is supposed to fit. Inaccurate sizing can, in turn, result in damage to the probe.
Some drill collars include inwardly-projecting centralizing features designed to protect downhole probes. For example, U.S. Pat. No. 5,520,246 discloses apparatus for protecting instrumentation placed within a drill string. The apparatus includes multiple elastomeric pads spaced about a longitudinal axis and protruding in directions radially to the axis. US 2005/0217898 describes a drill collar having a longitudinal axis and an inner surface facing the longitudinal axis. Multiple elongate ribs are mounted to the inner surface and extend parallel to the longitudinal axis.
Since well drilling can be exceedingly expensive, it may be required to have at the well site a spare probe and a spare set of drill collars to support the probe. This can represent an undesirably large capital outlay and also large costs for transporting the probes and associated sets of collars to the well site. Some probes are 15 meters long or more. Drill collars of 11 inches or more in diameter are not uncommon.
There is a need for a better way to provide downhole probes for use in drill strings especially where it is desired to use the same probe in drill string sections of different diameters.